The present application claims priority to Japanese Application(s) No(s). P2001-029946 filed Feb. 6, 2001, and P2001-070650 filed Mar. 13, 2001, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a magnetic tunnel element in which is formed a ferromagnetic tunnel junction for use in magnetic devices such as a thin-film magnetic head for reading out (reproducing), for example, high density recordings and a magnetic sensor for sensing a magnetic field and a method of manufacturing such a magnetic tunnel element. More particularly, this invention relates to a magnetic tunnel element, a manufacturing method of such a magnetic tunnel element and a thin-film magnetic head, a magnetic memory and a magnetic sensor, each using such a magnetic tunnel element.
2. Description of the Related Art
It is known that, if an electrical insulator has a thickness ranging from several angstroms to several tens of angstroms in a junction having an arrangement in which a metal, an electrical insulator and a metal are laminated, when a voltage is applied to the upper and lower metals which sandwich the electrical insulator, a very small current can flow through the electrical insulator.
This phenomenon is referred to as a xe2x80x9ctunnel effectxe2x80x9d and can be expressed from the standpoint of quantum mechanics.
A current which flows through the electrical insulator is called a xe2x80x9ctunnel currentxe2x80x9d.
The junction having the structure of the ferromagnetic metal/electrical insulator/ferromagnetic metal in which the upper and lower metals which sandwich the electrical insulator are made of ferromagnetic materials is referred to as a xe2x80x9cferromagnetic tunnel junctionxe2x80x9d.
In the case of this ferromagnetic tunnel junction, the magnitude of a tunnel current depends upon the magnetization states of the upper and lower ferromagnetic metal layers. Specifically, when the directions of the two magnetizations are the same, the largest tunnel current can flow. On the other hand, when the directions of the two magnetizations are opposite to each other, a tunnel current decreases.
It has been explained that this phenomenon is caused when conduction electrons are polarized and the conduction electrons tunnel while keeping the polarization. Specifically, electrons polarized in one direction can tunnel in the polarized direction and electrons polarized in the opposite direction can tunnel in the opposite direction.
For this reason, when the magnetization directions of the upper and lower ferromagnetic metals which sandwich the insulating layer are the same direction, electrons can tunnel from the same state to the same state so that a large tunnel current can flow.
On the other hand, when the magnetization directions of the upper and lower ferromagnetic metals which sandwich the insulating layer are opposite to each other, if electrons polarized in one direction and electrons polarized in the opposite direction have no spaces for tunneling, then electrons cannot tunnel in either direction. As a result, in general, a tunnel current decreases (tunnel probability decreases).
Accordingly, there occurs a so-called magnetic tunnel effect in which the conductance of a tunnel current which flows through the insulating film varies depending upon relative angles of the magnetization directions of the upper and lower ferromagnetic metals.
Specifically, the laminated structure in which the thin insulating layer is sandwiched between the magnetic layers comprised of the pair of ferromagnetic metals may demonstrate a magnetoresistive effect against the tunnel current which flows in the insulating layer.
In this magnetic tunnel effect, a magneto-resistance changing ratio, i.e., magnetoresistive ratio (MR ratio) can be theoretically calculated by a polarizability of the magnetization of the magnetic layers comprised of the pair of ferromagnetic metals.
In particular, when the pair of magnetic layers are made of CoFe, a magnetoresistive ratio of about 50% can be expected, and hence there can be obtained a magnetoresistive ratio larger than an anisotropic magnetoresistive effect (AMR effect) and a giant magnetoresistive effect (GMR effect).
Therefore, the magnetic tunnel junction element having the laminated structure in which the thin insulating layer is sandwiched between the pair of magnetic layers (hereinafter referred to as a xe2x80x9cTMR elementxe2x80x9d) receives remarkable attention as a magnetoresistive effect element. Thus, it can be expected that this TMR element may be applied to a next generation magnetic head, a next generation magnetic sensor and further a next generation magnetic memory.
In particular, in the field of the magnetic heads, a so-called magnetic tunnel effect-type magnetic head which uses this TMR element as a magnetic sensing element for detecting a magnetic signal from a magnetic recording medium receives remarkable attention.
This TMR head is a shield type TMR head in which a TMR element is disposed between a pair of magnetic shield layers through a gap layer. When this pair of magnetic shield layers are given functions as electrodes, a gap between the pair of magnetic shield layers and the TMR element can be narrowed.
However, in the TMR head, there arises a phenomenon in which a magnetoresistive ratio (hereinafter referred to as an xe2x80x9cMR ratioxe2x80x9d) is lowered when a bias voltage is applied to the TMR element, i.e., so-called bias voltage dependency of a magnetic tunnel element.
FIG. 1 shows this bias voltage dependence as data in actual practice. A study of FIG. 1 reveals that the magnetoresistive ratio (MR ratio) is lowered when an applied voltage is increased in the positive (+) direction or in the negative (xe2x88x92) direction.
The phenomenon of this bias voltage dependency is a characteristic inherent in the TMR element, and there arises a problem of how to alleviate this bias voltage dependence when a TMR head is fabricated into a device.
The problem of this bias voltage dependency becomes very important when the TMR element is applied not only to a magnetic head but also to a magnetic sensor and a magnetic memory.
For the purpose of solving the aforesaid problems, it is an object of the present invention to provide a magnetic tunnel element in which a magnetoresistive ratio can be suppressed from being lowered by decreasing a bias voltage dependency so that the magnetic tunnel element becomes highly reliable and also becomes able to obtain a high output when this magnetic tunnel element is applied to a thin-film magnetic head, and a method of manufacturing such magnetic tunnel element. Another object of the present invention is to provide a thin-film magnetic head, a magnetic memory and a magnetic sensor including the above-mentioned magnetic tunnel element and which can obtain a high output and which become highly reliable.
According to an aspect of the present invention, there is provided a magnetic tunnel element which is comprised of a plurality of ferromagnetic layers and an insulating film formed of metal oxide films, wherein the plurality of ferromagnetic element are laminated across the insulating film and the asymmetric tunnel barriers are formed by the insulating film along the laminated direction.
According to another aspect of the present invention, there is provided a method of manufacturing a magnetic tunnel element comprising a plurality of ferromagnetic films and an insulating film formed of metal oxide films wherein a plurality of ferromagnetic films are laminated across the insulating film. This method of manufacturing a magnetic tunnel element is comprised of at least the processes of depositing a metal film and forming a first metal oxide film by oxidizing the metal film and depositing a metal film on the first metal oxide film and forming a second metal oxide film by oxidizing the metal film under oxidation conditions different from the process for forming the first metal oxide film, thereby to form the insulating film.
According to a further aspect of the present invention, there is provided a thin-film magnetic head which is comprised of a magnetic tunnel element including a plurality of ferromagnetic films and an insulating film formed of metal oxide films, wherein a plurality of ferromagnetic films are laminated across the insulating film and asymmetric tunnel barriers are formed by the insulating film along the direction in which a plurality of ferromagnetic layers are laminated and upper and lower magnetic shields laminated across the magnetic tunnel element.
In accordance with a yet further aspect of the present invention, there is provided a magnetic memory which is comprised of a bit line, a word line and a magnetic tunnel element including a plurality of ferromagnetic films and an insulating film formed of metal oxide films wherein a plurality of ferromagnetic films are laminated across the insulating film and asymmetric tunnel barriers are formed along the direction in which a plurality of ferromagnetic films are laminated by the insulating film, wherein a memory cell including the magnetic tunnel element is disposed corresponding to an intersection between the bit line and the word line.
In accordance with a still further aspect of the present invention, there is provided a magnetic sensor which is comprised of a magnetic tunnel element including a plurality of ferromagnetic films and an insulating film formed of metal oxide films, wherein an asymmetric barrier is formed along the direction in which a plurality of ferromagnetic films are laminated by the insulating film and upper and lower magnetic shields are laminated across the magnetic tunnel element.
According to the arrangement of the magnetic tunnel element of the present invention, since the asymmetric tunnel barriers are formed by the insulating film along the direction in which the ferromagnetic films are laminated, the magnetoresistive ratio of the magnetic tunnel element can be suppressed from being lowered due to the bias voltage (bias voltage dependency), thereby making it possible to maintain a high magnetoresistive ratio even under the condition that the bias voltage is applied to the magnetic tunnel element.
According to the above-mentioned manufacturing method of the present invention, since the second metal oxide film is formed by oxidizing the metal film under oxidation conditions different from those of the first metal oxide film, the degree to which the first metal oxide film is oxidized and the degree to which the second metal oxide film is oxidized can be made different from each other. Thus, the insulating barriers having different heights can be formed by the first metal oxide film and the second metal oxide film so that the asymmetric insulating barrier can be formed in the insulating film of the magnetic tunnel element along the direction in which the films are laminated.
According to the arrangement of the magnetic head of the present invention, since the above magnetic head includes the above magnetic tunnel element of the present invention, the magnetoresistive ratio of the magnetic tunnel element can be suppressed from being lowered due to the bias voltage. As a result, under the condition that the bias voltage is applied to the magnetic tunnel element in order to detect whether or not the magnetoresistive is changed by a magnetic field from a recording layer of a recording medium (magnetic recording medium), it becomes possible to maintain a high output based upon the change of the magneto-resistance.
Further, according to the above arrangement of the magnetic memory of the present invention, since the magnetic memory includes the above magnetic tunnel element of the present invention, the magnetoresistive ratio of the magnetic tunnel element can be suppressed from being lowered due to the bias voltage. As a result, under the condition that the bias voltage is applied to the magnetic tunnel element in order to record information on the magnetic memory or to read out information from the magnetic memory, it becomes possible to maintain a high output based upon the change of the magneto-resistance.
Furthermore, according to the above-mentioned arrangement of the magnetic sensor of the present invention, since the magnetic sensor includes the above-mentioned magnetic tunnel element of the present invention, the magnetoresistive ratio of the magnetic tunnel element can be suppressed from being lowered due to the bias voltage. As a result, under the condition that the bias voltage is applied to the magnetic tunnel element in order to detect whether or not the magneto-resistance is changed due to an external magnetic field, it becomes possible to maintain a high output based upon the change of the magneto-resistance.